The present invention relates generally to photocathodes, to devices incorporating photocathodes, and to methods of making photocathodes.
A photocathode is an opto-electronic device that emits electrons when it is struck by photons of light. A photocathode typically is used in a vacuum tube with an anode structure. In the simplest case, the anode structure can be a simple plate, where electrons emitted from the photocathode pass directly to the anode structure. In this simple structure, the current passing through the device is essentially equal to the number of electrons emitted by the photocathode. In more complex structures, e.g., photomultiplier tubes, the anode structure includes known electron-multiplying devices such as microchannel plates and dynodes. These devices act to emit large numbers of electrons in response to a few electrons emitted by the photocathode. In such a tube, the current passing through the device is many times greater than the emission current from the photocathode, i.e., the number of electrons emitted from the photocathode. In all of these structures, however, the current passing through the device is directly related to the emission current from the photocathode. Thus, to make a device that is highly sensitive, it is desirable to use a photocathode with a high quantum efficiency, i.e., a photocathode which emits a relatively large number of electrons for a given amount of light impinging on the photocathode. This is especially desirable where the device is used to detect extremely dim light as, for example, in so-called “single photon detectors” used in certain scientific applications. A single photon detector is intended to provide a measurable electrical current in response to a single photon impinging on the photocathode.
Conventional photocathodes are formed by depositing a layer of polycrystalline material on a planar substrate. The layer of polycrystalline material forms the photoemissive surface for absorbing the light and releasing the electrons. The photoemissive surface of a conventional photocathode is relatively smooth and of the same size as the underlying substrate. The planar substrate is formed of an electrically conductive material and is electrically coupled to the polycrystalline layer.
The quantum efficiency of a photocathode is the ratio of the number of released electrons over the number of incident photons of a given wavelength. The maximum efficiency is 100%. Conventional photocathodes have a peak quantum efficiency of 25% at a materials dependent wavelength between the range of 200 nm-900 nm. This means that approximately 75% of the incident photons do not cause emission of electrons.
There exists a need for photocathodes that have high quantum efficiency and for devices incorporating such photocathodes. There also exists a need for a method to manufacture these high quantum efficiency photocathodes.